Sleep is generally beneficial and restorative to a patient, exerting great influence on the quality of life. The human sleep/wake cycle generally conforms to a circadian rhythm that is regulated by a biological clock. Regular periods of sleep enable the body and mind to rejuvenate and rebuild. The body may perform various tasks during sleep, such as organizing long term memory, integrating new information, and renewing tissue and other body structures.
Normal sleep is characterized by a general decrease in metabolic rate, body temperature, blood pressure, breathing rate, heart rate, cardiac output, sympathetic nervous activity, and other physiological functions. However, studies have shown that the brain's activity does not decrease significantly during sleep. Normally a patient alternates between rapid eye movement (REM) and non-REM (NREM) sleep in approximately 90 minute cycles throughout a sleep period. A typical eight hour sleep period may be characterized in terms of a five-step sleep cycle identifiable through brain wave activity.
Non-REM sleep includes four sleep states or stages that range from light dozing to deep sleep. Throughout NREM sleep, muscle activity is still functional, breathing is low, and brain activity is minimal. Approximately 85% of the sleep cycle is spent in NREM sleep. Stage 1 NREM sleep may be considered a transition stage between wakefulness and sleep. As sleep progresses to stage 2 NREM sleep, eye movements become less frequent and brain waves increase in amplitude and decrease in frequency. As sleep becomes progressively deeper, the patient becomes more difficult to arouse. Stage 3 sleep is characterized by 20 to 40% slow brain wave (delta) sleep as detected by an electroencephalogram (EEG). Sleep stages 3 and 4 are considered to be the most restful sleep stages.
REM sleep is associated with more prevalent dreaming, rapid eye movements, muscle paralysis, and irregular breathing, body temperature, heart rate and blood pressure. Brain wave activity during REM sleep is similar to brain wave activity during a state of wakefulness. There are typically 4-6 REM periods per night, with increasing duration and intensity toward morning. While dreams can occur during either REM or NREM sleep, the nature of the dreams varies depending on the type of sleep. REM sleep dreams tend to be more vivid and emotionally intense than NREM sleep dreams. Furthermore, autonomic nervous system activity is dramatically altered when REM sleep is initiated.
Lack of sleep and/or decreased sleep quality may be have a number of causal factors including, e.g., nerve or muscle disorders, respiratory disturbances, and emotional conditions, such as depression and anxiety. Chronic, long-term sleep-related disorders e.g., chronic insomnia, sleep-disordered breathing, and sleep movement disorders, including restless leg syndrome (RLS), periodic limb movement disorder (PLMD) and bruxism, may significantly affect a patient's sleep quality and quality of life.
Movement disorders such as restless leg syndrome (RLS), and a related condition, denoted periodic limb movement disorder (PLMD), are emerging as one of the more common sleep disorders, especially among older patients. Restless leg syndrome is a disorder causing unpleasant crawling, prickling, or tingling sensations in the legs and feet and an urge to move them for relief. RLS leads to constant leg movement during the day and insomnia or fragmented sleep at night. Severe RLS is most common in elderly people, although symptoms may develop at any age. In some cases, it may be linked to other conditions such as anemia, pregnancy, or diabetes.
Many RLS patients also have periodic limb movement disorder (PLMD), a disorder that causes repetitive jerking movements of the limbs, especially the legs. These movements occur approximately every 20 to 40 seconds and cause repeated arousals and severely fragmented sleep.
A significant percentage of patients between 30 and 60 years experience some symptoms of disordered breathing, primarily during periods of sleep. Sleep disordered breathing is associated with excessive daytime sleepiness, systemic hypertension, increased risk of stroke, angina and myocardial infarction. Disturbed respiration can be particularly serious for patients concurrently suffering from cardiovascular deficiencies. Disordered breathing is particularly prevalent among congestive heart failure patients, and may contribute to the progression of heart failure.
Sleep apnea is a fairly common breathing disorder characterized by periods of interrupted breathing experienced during sleep. Sleep apnea is typically classified based on its etiology. One type of sleep apnea, denoted obstructive sleep apnea, occurs when the patient's airway is obstructed by the collapse of soft tissue in the rear of the throat. Central sleep apnea is caused by a derangement of the central nervous system control of respiration. The patient ceases to breathe when control signals from the brain to the respiratory muscles are absent or interrupted. Mixed apnea is a combination of the central and obstructive apnea types. Regardless of the type of apnea, people experiencing an apnea event stop breathing for a period of time. The cessation of breathing may occur repeatedly during sleep, sometimes hundreds of times a night and occasionally for a minute or longer.
In addition to apnea, other types of disordered respiration have been identified, including, for example, hypopnea (shallow breathing), dyspnea (labored breathing), hyperpnea (deep breathing), and tachypnea (rapid breathing). Combinations of the disordered respiratory events described above have also been observed. For example, Cheyne-Stokes respiration (CSR) is associated with rhythmic increases and decreases in tidal volume caused by alternating periods of hyperpnea followed by apnea and/or hypopnea. The breathing interruptions of CSR may be associated with central apnea, or may be obstructive in nature. CSR is frequently observed in patients with congestive heart failure (CHF) and is associated with an increased risk of accelerated CHF progression.
An adequate duration and quality of sleep is required to maintain physiological homeostasis. Untreated, sleep disturbances may have a number of adverse health and quality of life consequences ranging from high blood pressure and other cardiovascular disorders to cognitive impairment, headaches, degradation of social and work-related activities, and increased risk of automobile and other accidents.